Microbial processes play a central role in wastewater management. In particular, they underpin biological treatment of wastewater, the most cost-effective and environmentally friendly method for wastewater treatment.
A typical advanced wastewater treatment plant receives wastewater from sewage mains. The wastewater is first treated to remove large particulates (by screening, or passing through a primary settler, or both). The liquor then passes to bioreactors, where bacteria mineralise organic carbon (often referred to as biological oxygen demand or BOD) to CO2 and convert ammonia to nitrate, and in some cases further to nitrogen gas. Some bioreactors also achieve biological phosphorus removal. This process results in the growth of biomass. The biomass is then separated from the liquor, typically in a secondary settler.
The sludge from the secondary settler (which includes most of the separated biomass) is then treated in an anaerobic digester or an aerobic digester, sometimes together with primary sludge resulting from the settling process in the primary settler. In the anaerobic digester, the BOD of the sludge is converted to methane. Products from the anaerobic digester also include solids that may be disposed of and a liquid stream. In the aerobic digester, part of the organics in the sludge is mineralised thus achieving the stabilisation and a reduction of the sludge to be disposed of.
Variations around this general process described above also exist.
Bioreactors used for treating primary effluent can consist of aerobic, anoxic and even anaerobic zones/conditions. Throughout this specification, the term “bioreactor for treating wastewater” is used to refer to any reactor in which microorganisms utilise or catalyse conversion of wastewater stream components into other components. The bioreactor may be an aerobic bioreactor, an anaerobic bioreactor or an anoxic bioreactor, or it may be operated under two or more such conditions (typically in sequence, but different zones of a bioreactor may operate under different conditions, for example, a top part of a bioreactor may be operating under aerobic conditions and a bottom part of the bioreactor may be operating under anaerobic conditions.
In a typical wastewater treatment plant, both biological nutrient removal and energy recovery require organic carbon. The requirement for high-level nutrient removal from wastewaters has often seen the abolishment of the primary settler, to satisfy the carbon demand for nutrient removal in the downstream processes of the wastewater treatment plant. However, abolishing the primary settler eliminates an energy rich stream for anaerobic digestion. This reduces the energy yield of the plant and renders energy recovery through anaerobic digestion economically infeasible for small to medium-sized wastewater treatment plants.
One reason for the high demand of organic carbon feed for nutrient removal is biomass production. In this regard, in the bioreactor for treating wastewater, the reactions that are taking place are typically biologically driven. As a result, the microorganisms that catalyse these reactions grow and a substantial biomass is produced. These microorganisms assimilate a large amount organic carbon as biomass. Typically, 30 to 40% of the organic carbon fed to the bioreactor is assimilated by bacterial cells in the form of active bacterial cells and debris resulting from cell death and lysis, and is subsequently removed from the bioreactor as excess secondary sludge.
The secondary sludge is often supplied to an anaerobic digester in order to convert the BOD of the sludge to biogas containing methane. However, this large stream of secondary sludge, although containing large amounts of organic carbon, is poorly biodegradable. Pre-treatment of the sludge is required to break up bacterial cell walls to make its carbon more available for the reactions in the anaerobic digester, such as methane production, or in another bioreactor for treating wastewater as an external carbon source for denitrification.
Various methods have been developed to improve the bioavailability of this sludge stream. However, these methods are either energy intensive (such as thermal treatment, sonication, or ozonation) or consume large amounts of imported chemicals, such as acid, alkali or hydrogen peroxide. This incurs significant economic and environmental costs.